Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that improves power delivery performance and fuel economy by achieving at least nine forward speed stages and widening speed ratio (or gear ratio) with a minimum number of constituent elements being used, and secures linearity of step ratios.
Description of Related Art
Recent increases in oil prices are triggering hard competition among auto-makers in enhancing fuel consumption of a vehicle.
In this sense, research is being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and research is also being conducted to ensure both drivability and competitiveness for maximizing fuel efficiency by implementing an automatic transmission with multiple speed stages.
However, in the case of the automatic transmission, the number of internal components increases as the number of gear shift stages is increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift-stages, it is important that better efficiency is achieved using a smaller number of parts.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, since a conventional 8-speed automatic transmission has gear ratio span of 6.5-7.5, improvement of power performance and fuel economy may not be great.
In addition, if 8-speed automatic transmission has gear ratio span larger than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speed stages is necessary.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.